My 2006 ml350 radio stopped working abruptly. what is the problem? can i get a command system separately?

SOURCE: installing back up camera

First, your ML's COMAND head MUST be pre-installed to back up camera. Check with a voltmeter the plugs no. 1 and 7 in the back of the COMAND, they must have power. If not, you'll need a COMAND head with back up camera option.
If yes, you are free to buy a MB back up camera, install and plug and have a dealer encode your MOST fiber ring to let the car know you are using a BUC.
If you plan to use an after market BUC, like Pioneer's, you'll need to buy an adaptor. Expensive tho.
Good luck.